In C-clamps, vises, and other clamping devices, a screw threaded shank moves a clamping device such as a pressure pad, a vise jaw, etc. Turning of the threaded shank advances the clamping device thread-by-thread. However, in large C-clamps, large vises, or any clamping device having a large throat it is desirable to move the threaded shank axially over a substantial distance rapidly without having to turn the shank and advance it thread-by-thread.
Devices for attaining this end are known in which a nut or other structure in which the shank turns has only partial threads which the shank may engage for advancing thread-by-thread, or from which the threaded shank may be disengaged for axial movement of the shank rapidly without regard for the threads. One such device is shown in Farrell U.S. Pat. No. 2,430,458. Various problems were found in practice with the device disclosed in the aforesaid Farrell patent. Both the C-shaped frame and the partially threaded nut were made of cast iron. It was found in practice that the cast iron would sometimes fail, either through fracture of the C-shaped frame, or through sudden stripping of the partial threads from the nut. This resulted in considerable danger to workers in the area as well as to the immediate user, since large clamps are frequently used for hoisting and conveying rather heavy articles. Failure of either the C-shaped frame or the nut resulted in dropping a part so carried with considerable danger to those in the area.
Furthermore, the cross-sectional shape of the nut was generally oval, consisting of arcs of different radii blending smoothly into one another. The shape of the housing for the nut was semi-cylindrical, with parallel walls extending from the semi-cylinder. As a result, there were large areas in contact which presented frictional or rubbing problems which could result in erratic and unpredictable operation, or jamming of the nut within the housing. Although friction is theoretically independent of area, in actual practice the surfaces of cast members tend to be rather rough, whereby there is a resistance to moving of one surface over the other which is considerably in excess of the actual frictional resistance. Minor protuberances and declevities of confronting surfaces often tend to lock with one another to present a rather substantial resistance to relative movement.